The present invention relates to the field of hydrocarbon production processes and, in particular, to steam-based hydrocarbon production processes.
Hydrocarbons are recovered in conventional processes using fluids such as steam or solvent. For example, steam has been used in a Steam Assisted Gravity Drainage (SAGD) process as described in U.S. Pat. No. 4,344,485 (Butler, Aug. 17, 1982) and solvent has been used in a solvent-assisted gravity drainage process as described in U.S. Pat. No. 5,899,274 (Frauenfeld et al, May 4, 1999).
A combination of solvent with steam or hot water has also been proposed for increasing hydrocarbon production from subterranean formations in, for example, U.S. Pat. No. 4,753,293 (Bohn, Jun. 28, 1988) and U.S. Pat. No. 4,884,635 (McKay et al., Dec. 5, 1989).
Bohn describes a solvent-based hydrocarbon recovery process using in-situ condensation of hydrocarbon/water vapor mixture to simultaneously heat and dissolve tar in heavy oil. But the mixed vapor is a mixture of at least 40 wt. % (about 52 vol. %) hot hydrocarbon-solvent vapors and from 5 to 60 wt. % water vapor (i.e., steam) Accordingly, the volume ratio of solvent to steam in Bohn""s solvent-based mixture is greater than 1.1:1 or, put another way, the steam to solvent volume ratio is less than 0.9:1. Bohn suggests maintaining the mixed solvent-to-oil ratio (xe2x80x9cMSORxe2x80x9d) in the range of 2:1 to about 10:1. Assuming 85% solvent recovery in the produced oil, Bohn suggests that, to produce 1 barrel of oil, his process requires from 4.5 barrels injected fluid (2.4 barrels solvent+2.1 barrels steam) to 22.4 barrels injected fluid (11.8 barrels solvent+10.6 barrels steam) when the solvent to steam ratio is 1.1:1.
Bohn teaches that if produced liquids become more viscous than desired, indicating an undesirably low MSOR in the mixing zone, the condition should be corrected either by increasing the solvent to water ratio of incoming vapor (i.e., decreasing water-to-solvent ratio of incoming vapor) or by decreasing the extent to which incoming solvent is superheated (i.e., its enthalpy in excess of P, T saturation). Accordingly, Bohn suggests that one way to correct a low MSOR is to further increase the amount of solvent injected in his solvent-based process.
U.S. Pat. No. 4,884,635 (McKay et al., Dec. 5, 1989) describes a hot water-based hydrocarbon recovery process using a mixture of hot water and hydrocarbon additive. The concentration of hydrocarbon liquid additive present in the injection mixture is in a range of about 0.1 wt. % to about 10 wt. %. Also, the aromatic content of the hydrocarbon liquid is greater than about 30%. McKay et al. teach decreasing oil recovery with increased hydrocarbon additive concentrations. Specifically, McKay et al. teach xe2x80x9can insubstantial improvement over the hot water flood without additivexe2x80x9d when the hydrocarbon additive concentration was increased to 10 wt. % (col. 3:45-47). Accordingly, McKay et al. teach improved oil recovery with a minor amount of hydrocarbon additive, but oil recovery decreased as the hydrocarbon additive concentration increased to 1 wt. %, 5 wt. % and finally 10 wt. %.
Accordingly, those skilled in the art have understood from Bohn""s and McKay""s processes that increased oil recovery is generally obtained by either increasing the volume of the solvent in a solvent-based process or increasing the volume of steam/hot water in a steam- or hot water-based process.
Alternating solvent and steam or water injections have also been described, for example, in related patents U.S. Pat. No. 3,954,141 (Allen et al., May 4, 1976) and U.S. Pat. No. 4,004,636 (Brown et al., Jan. 25, 1977).
Allen et al. describe a multi-component hydrocarbon solvent process using a miscible gaseous/liquid hydrocarbon solvent mixture to reduce the viscosity of the oil in place for improving the oil""s displacement. A slug of the solvent mixture is injected into the formation, so that substantially all of the solvent mixture is in the liquid phase. Solvent mixture injection is terminated and then the pressure in the formation is reduced to vaporize at least a portion of the gaseous component in the solvent mixture. Later in the process, an inexpensive inert drive fluid, such as water, is injected to displace the solvent slug further from the wellbore to achieve the desired contact between solvent and oil in place. The steps are then repeated, with the gaseous/liquid solvent mixture displacing the inert drive fluid and vice versa. Examples of gaseous hydrocarbon solvents include methane, ethane, propane, butane, pentane, ethylene, propylene, butylene and carbon dioxide. Examples of liquid hydrocarbon solvents are hexane, heptane and higher molecular weight aliphatic and aromatic hydrocarbons.
In the related Brown et al. patent, also disclosing a multi-component solvent process using a gaseous/liquid hydrocarbon solvent, Brown et al. teach that, after oil is produced with the solvent mixture, a hot fluid, such as steam or a mixture of steam and solvent, is injected into the formation. The hot fluid is at a temperature substantially greater than the boiling point of the gaseous component of the solvent mixture. The hot fluid causes the gaseous component to vaporize, which results in a volume expansion. In turn, the volume expansion produces a drive pressure that helps drive the oil in place toward the production well. Steam injection is continued until at least a major portion of the formation being swept exceeds the vaporization temperature of the gaseous solvent, at which point steam injection is terminated and water is injected to finish displacing oil with the solvent mixture toward the production well.
Though Brown et al. suggest injecting a steam/solvent mixture as a heated fluid to vaporize the gaseous component of the solvent slug previously injected, there is no suggestion for the relative proportions of steam and solvent to use in the steam/solvent mixtures used for vaporizing the solvent mixture previously injected. And, though there may be some limited mixing at the interface between solvent and steam or water slugs, neither Brown et al. nor Allen et al. suggest how relative proportions of solvent to steam or hot water at the interface could be altered so that either less solvent and/or less steam may be used, while recovering significant volumes of oil. Moreover, both Allen et al. and Brown et al. stress the need for a solvent recovery process in order to reduce the quantity of solvent required in their process.
Accordingly, there is a need for an improved oil production process that can increase the volume of hydrocarbon produced per unit volume of steam and/or solvent injected. Also, there is a need for an improved oil production process that can reduce the reliance on an auxiliary solvent recovery process for making use of a water/steam and solvent injection fluid economically feasible.
According to one aspect of the present invention, there is provided a predominantly water-based thermal method for producing hydrocarbons from a subterranean formation having indigenous hydrocarbons, at least one producing means that can communicate with at least a portion of the formation, at least one injection means comprising a wellbore, and a mixing zone in a near-wellbore region of the wellbore, the method comprising:
(a) selecting a first component, W, and a second component, S, wherein component W is selected from steam, hot water and combinations thereof, and component S is a solvent selected from C1 to C30 hydrocarbons, carbon dioxide, carbon monoxide and combinations thereof;
(b) introducing at least component W and component S into the at least one injection means;
(c) preparing a heated water-based injection fluid composition before, in or after the at least one injection means, the heated water-based injection fluid composition comprising at least component W and component S in a W to S volume ratio, defined by:             (              W        ⁢                  :                ⁢        S            )        n    =                                          (                          volume              ⁢                              xe2x80x83                            ⁢              of              ⁢                              xe2x80x83                            ⁢              component              ⁢                              xe2x80x83                            ⁢              W              ⁢                              xe2x80x83                            ⁢              injected              ⁢                              xe2x80x83                            ⁢              in                                                                                      an              ⁢                              xe2x80x83                            ⁢              injection              ⁢                              xe2x80x83                            ⁢              sequencing              ⁢                              xe2x80x83                            ⁢              interval              ⁢                              xe2x80x83                            ⁢              n                        )                                                                                  (                              volume                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                component                ⁢                                  xe2x80x83                                ⁢                S                ⁢                                  xe2x80x83                                ⁢                injected                ⁢                                  xe2x80x83                                ⁢                in                                                                                                        an                ⁢                                  xe2x80x83                                ⁢                injection                ⁢                                  xe2x80x83                                ⁢                sequencing                ⁢                                  xe2x80x83                                ⁢                interval                ⁢                                  xe2x80x83                                ⁢                n                            )                                          ⁢              xe2x80x83            
xe2x80x83where the total number of injection sequencing intervals is in a range from about 2 to about 12,000,
xe2x80x83wherein the heated water-based injection fluid composition has a first W to S volume ratio, (W:S)1, greater than about 5:1 and the volume of component S injected in the first injection sequencing interval, where n=1, is greater than 0;
(d) changing the ratio, at least once, from (W:S)1 to a different W to S volume ratio, (W:S)n, wherein at least one (W:S)n is less than (W:S)1 and each (W:S)n is greater than or equal to about 1.5:1; and
(e) collecting at least a portion of the indigenous hydrocarbons using the at least one producing means.